Water filtration systems are commonly employed for filtering high volumetric flow rates of liquids, e.g., in municipal and industrial water treatment and waste water treatment plants. Each filtration system generally comprises a filter basin having a floor and vertical walls surrounding the floor, and an underdrain assembly positioned over the floor. A bed of granular filter media for filtering a liquid as it seeps downward through the filter bed is placed on top of the underdrain assembly. The underdrain assembly defines a perforated false bottom in the basin for supporting the filter media bed and for providing a plurality of fluid passageways for both removing the filtered water from the bottom of the filter basin and directing water and/or air into the filter bed during backwashing.
The filter media bed is generally several feet deep and is typically comprised of successive layers of gravel, sand, anthracite, or other granular filter media. The filter media bed for both traditional and gravel-less designs typically transition from coarse sizes near the top of the filter media bed to fine sizes near the bottom. Traditional filter designs place support media, such as multiple gravel layers, with relatively coarse sizes between the fine filter media positioned near the bottom of the filter media bed and the top surface of the underdrain assembly to prevent the finer filter media from entering the underdrain apertures and contaminating the filtered water. Gravel-less filter designs do not use support media, but rather use various types and configurations of porous filters to prevent the granular filter media from entering the underdrain assembly apertures.
During operation of the filtration system, unfiltered water is directed into the filter basin to a depth of several feet above the upper layer of filter media and flows downward though the filter media bed. During this filtration process, suspended materials in the unfiltered water become trapped in the filter media. The water ultimately reaches the bottom of the filter bed and passes through apertures in the underdrain assembly. The water is then collected in passageways within the underdrain assembly and is carried out of the filter basin through a suitable conduit or flume.
After the filtration system is operational for a period of time, the efficiency of the system decreases and it becomes necessary to wash the filter media bed to remove material trapped therein. A backwashing process commonly is utilized to flush the filter media of trapped particles. The backwashing process involves pumping pressurized water and/or air in a reverse direction into the underdrain assembly passageways, through apertures formed in the underdrain assembly, and into the overlying filter media bed. The backwash water flows through the filter media bed and carries the trapped materials upward through the filter bed. The wash water and the materials entrained or suspended therein are then collected at the top of the filter basin and carried away.
During the backwashing operation it is desirable to obtain a uniform distribution of wash water throughout the filter media bed to completely wash the entire filter bed. If the wash water distribution is uneven so that dead spots occur at certain locations within the filter bed, then those portions of the filter bed will be improperly cleansed, thereby reducing the efficiency of the filter. The backwashing process must also be performed under carefully controlled conditions so as to avoid unduly disturbing or damaging the filter media bed. For example, the velocity of the wash water must be controlled at a level below that which would cause the filter media to become entrained in the wash water along with the removed materials and carried away as waste. Explosive bursts of backwash water that open channels in the filter media, typically occurring at the initiation of the backwashing cycle, must be avoided. These open channels allow unfiltered water to pass through the filter media without being filtered and allow finely-sized filter media to be carried away with the filtered water.